The question of the significance and nature of biological differences observed between isolates of human cytomegalovirus (CMV) is being investigated. Using strain-specific rabbit antisera additional evidence indicating a degree of antigenic heterogeneity has been obtained. Differences in plaque morphology occur and minimal differences occur in terms of temperature-permissiveness. A second area relates to the antigenic structure of the CMV. Early antigen production occurs in infected human fibroblasts inhibited with cytosine-arabinoside. A third area deals with cellular immunity. Lymphocytes from sero-positive donors undergo blastogenesis on incubation with CMV antigens; a good correlation exists between the results of studies on humoral and on cellular immunity. A different area of research is the continued effort to obtain growth of schistosomal cells in vitro. In the best of the media examined to date, scattered nests of non-replicating cells migrate and persist for considerable periods of time. BIBLIOGRAPHIC REFERENCE: Albrecht, T., and Weller, T.H. The biological diversity of human cytomegalovirus isolates. (Abstract) Abstracts, Ann. Mtg. Am. Soc. Microbiol., p. 235, 1976.